In recent years, there is a demand for an improvement in exhaust emission of internal combustion engines provided in automobiles or the like. Particularly for diesel engines, removal of PM, such as soot or the like, included in exhaust gas is required in addition to a reduction in CO, HC and NOx. Therefore, a filter made of a porous material or the like is provided in an exhaust path of an engine so that PM in exhaust gas is collected by the filter.
Since the filter is made of a porous material or the like as described above, an excessive increase in the amount of collected PM (hereinafter also referred to as a PM accumulation amount) results in an increase in flow resistance in the filter, leading to a reduction in the output power of the engine or the like. Therefore, PM collected by the filter needs to be removed as appropriate, thereby restoring the filter to revitalize the PM collecting ability.
As a conventional filter restoring method, an operation of supplying backwash air into a filter or an operation of heating a filter using a heating apparatus is performed in a batch to remove PM as disclosed in, for example, Patent Document 1 (described below).
Also, a continuous restoration filter which can be continuously used so as to be applicable to automobile engines or the like, has been proposed in, for example, Patent Document 2. In Patent Document 2, a plurality of filters are connected in parallel, and some of the filters are used to collect PM while the other filters are subjected to a restoration operation, thereby making it possible to continuously operate the engine.
Since the continuous restoration filter has a larger size, a chemical reaction-type restoration technique which can have a smaller size also has been proposed (see, for example, Patent Document 3 described below). In this chemical reaction-type restoration technique, NO in exhaust gas is oxidized into NO2, and PM is removed by oxidation using 0 (oxygen) released when the NO2 returns to NO. For example, an oxidation catalyst, such as platinum or the like, is provided in a filter, and the oxidation action of the oxidation catalyst is utilized, thereby making it possible to restore the filter during the running of the engine.
However, in the chemical reaction-type restoration technique, the chemical reaction is not carried out unless the exhaust gas temperature is higher than or equal to a predetermined restoration operable temperature (e.g., 300° C.). In other words, when the exhaust gas temperature continues to be lower than the restoration operable temperature, a large amount of PM is accumulated in the filter, so that the filter is likely to be clogged. Therefore, when the accumulation amount of PM reaches a predetermined amount or more, the exhaust gas temperature needs to be increased to be higher than or equal to the restoration operable temperature by any means.
In view of this, in an engine comprising an electronically controlled pressure-accumulation fuel injector (e.g., a so-called common-rail injector), “post injection” that a fuel is injected again from the injector after a main fuel is injected and an expansion stroke is started, is performed so that the exhaust gas temperature is increased by combustion of the post injection fuel (see, for example, Patent Document 4 described below). Alternatively, an air intake throttling valve is provided for an air intake system, the opening degree of the throttling valve is reduced to decrease an intake air amount and thereby enrich the air-fuel ratio, thereby increasing a combustion temperature in a combustion chamber to increase the exhaust gas temperature (see, for example, Patent Document 5 described below).
Also, in a filter of the chemical reaction-type restoration technique, the PM accumulation amount of the filter needs to be correctly detected so as to appropriately obtain timing of starting the restoration operation.
In view of this, Patent Document 6 and Patent Document 7 described below have been proposed. In Patent Document 6, a pressure sensor is used to detect a pressure difference between an upstream side and a downstream side of a filter in an exhaust pipe. When the pressure difference reaches a predetermined value or more, it is determined that the PM accumulation amount has become large, so that a filter restoration operation is started. As the filter restoration operation, Patent Document 6 specifically discloses reducing the opening degree of an air intake throttling valve provided in an air intake system, reducing the opening degree of an exhaust throttling valve provided in an exhaust system, increasing a fuel injection amount, delaying a fuel injection time, and the like.
Also, Patent Document 7 discloses that a PM generation amount and a combustion rate constant corresponding to a running state of an engine are read out from a map, and the PM accumulation amount is estimated in accordance with a predetermined calculation expression.
Patent Document 1: JP H8-232639A
Patent Document 2: JP H11-236813A
Patent Document 3: JP 2001-271629A
Patent Document 4: JP H8-303290A
Patent Document 5: JP H6-137130A
Patent Document 6: JP H7-189654A
Patent Document 7: JP 2002-97930A